Generally, tire uniformity machines are used to test the characteristics of tires after production. This testing may include measurement of the dimensional characteristics of the tire and the forces on the tire at varying loads. To accomplish load testing, the tire is brought into the tire uniformity machine, mounted on a chuck, inflated and rotated by a drive assembly coupled to the spindle of the chuck while a load wheel is brought into contact with the periphery of the tire.
Previously, these drive assemblies have included a motor mounted on the tire uniformity machine at a distance from a spindle to which the load wheel or chuck was mounted. The motor is then coupled to the spindle by a belt or a chain to drive the spindle. For measurement purposes, a timing belt is typically used to couple the motor to the spindle. As is common in the art, the timing belt contains a number of cogs or teeth that mate with similar cogs on a cog wheel attached to the drive shaft of the motor. The fit between each cog is limited by machining tolerances introducing some error in the measurement of the spindle's rotation. Further, the cogs on the timing belt generate significant vibration as they are engaged, introducing additional error into the measurements taken at the tire.
These errors are exacerbated when either the cog wheel or timing belt have bad teeth. The presence of a bad tooth, i.e., one that is improperly sized or has been damaged or worn causing it to mesh imprecisely with mating teeth, may cause some slipping or other movement of the timing belt relative to the cog wheel or spindle and can often increase the magnitude of the vibration. The presence of a bad tooth is typically identified by a sudden increase in noise, often a growling sound, or by shaking created by operation of the machine. This increase occurs periodically as the bad tooth is engaged. As a practical matter, the vibration caused by the presence of teeth, which is increased when bad teeth are present, introduces error in the measurements taken by the tire uniformity machine. For instance, the teeth cause the rotational velocity of the spindle, as measured by the tire uniformity machine to appear not constant. In making the velocity measurement, the teeth on the timing belt cause ripples in the measured velocity, and bad teeth may cause a spike in this measurement.
In effect, errors created by the drive assembly essentially cause a false reading of the spindle's rotational velocity. Inasmuch as other measurements performed by the tire uniformity machine rely on the accurate measurement of these rotational velocities, the drive assembly errors migrate throughout the tire uniformity machine measurements, in effect, creating a false baseline on which further measurements are superimposed. As a consequence, the devices measuring the tire are actually measuring the tire as well as the motor thus preventing these devices from isolating the tire's characteristics.
As a separate matter, the prior art drive assemblies are bulky and less responsive in making changes in the rotational direction of the tire. As previously discussed, the typical drive assembly has a motor, cog wheel, and timing belt coupled to a spindle that drives the chuck. At times during the testing process, it is necessary to change the direction of rotation of the tire. In the majority of prior art system, machine tolerances, and the additional inertia of these components increases the amount of time necessary to reverse the motor and change the direction of the tire or loadwheel. While the period for changing the direction of the tire may be on the order of seconds or tenths of a second, these small periods accumulate with the large numbers of tires that are processed in a continuing production process in a given period of time. Reducing the time required to change direction during operation of the tire uniformity machine will result in the processing of a significant number of additional tires in a given period of time.